CE 473/573 Groundwater Fall 2012 Comments on homework 1 and 2

CE 473/573 Groundwater
Fall 2012
Comments on homework 1 and 2
1. Groups found interesting problems related to groundwater, including contamination
caused by hydraulic fracturing, transport of nutrients and contaminants in groundwater, and the effect of decreasing groundwater levels on untreated wooden piles. Make
sure to address the questions in the homework: What is the effect of the engineering
solution on the economy, environment, society, etc.? What might you like to learn in
CE 473/573 related to this problem?
2. Most groups estimated the water use of ethanol plants to be about 40 MGD, or 6% of
the total for Iowa. Some groups called that significant, while others said it is small.
A key part of the problem is explaining estimates and assumptions so that they can
be evaluated quantitatively. Most groups did that well.
3. As with problem 2, groups did well at explaining the estimates and assumptions.
4. As we discussed in class, designing an experiment often involves estimating what
you want to measure—in this case, the hydraulic conductivity of sandy loam—and
choosing a measurement method that has a scale large enough to cover the maximum
value without being too far on the low end that the uncertainty becomes too large.
For example, if you need to measure something that is 1 mm, a meter stick has a large
enough range, but the uncertainty is very large because the measurement is on the
order of the smallest division.
5. Most groups had the right idea: Because the soil properties do not change, the difference in conductivities is due to differences in density and dynamic viscosity. Then
one can write
ρw μoil
ρoil μw
The conductivity of oil is lower than water’s because of the increased resistance caused
by the higher viscosity. Remember too that oil and water are immiscible, at least in
laminar flow.
6. Be sure to check your calculations. Several groups had mistakes (e.g., wrong conversions, very large times) that could have been caught by the checker. Remember also
that although K has dimensions of velocity, it is not a velocity.
7. The only problem here was confusion over the difference between dynamic viscosity μ
and kinematic viscosity ν, which are related by ν = μ/ρ. The soil type suggested by
the conductivity differed from the soil type suggested by the grain size; that was your
instructor’s mistake.
8. Most groups realized that larger values of K and d maximize the Reynolds number
and therefore give a conservative (i.e., safe) estimate. Some groups did not answer the
question about whether Fetter’s recommendation works. For problems such as this
one, showing your numbers and calculations helps the TA and me to evaluate your
10. Most groups understood that the volume of water moving through the tube does not
change when only the area of the tube is changed and everything else is held constant.
For a given time, the volume of water is the product of the area of the tube and the
change in head. Therefore, when the area of the tube is increased, the change of head
is smaller.
11. A few groups somehow concluded that the flow increases in time. That prediction is
incorrect because the head is decreasing.